The present invention is concerned with semiconductor devices using epitaxial layers of mercury cadmium telluride. In the present application, the common chemical notations for mercury cadmium telluride, (Hg,Cd)Te or Hg.sub.1-x Cd.sub.x Te, will be used.
Mercury cadmium telluride is an intrinsic photodetector material which consists of a mixture of cadmium telluride, a widegap semiconductor (E.sub.g = 1.6eV), with mercury telluride, which is a semimetal having a "negative energy gap" of about 0.3eV. The energy gap of the alloy varies linearly with x, the mole fraction of cadmium telluride in the alloy. By properly selecting "x", it is possible to obtain mercury cadmium telluride detector material having a peak response over a wide range of infrared wavelengths.
(Hg,Cd)Te is of particular importance as a detector material for the important 8 to 14 micron atmospheric transmission "window." Extrinsic photoconductor detectors, notably mercury doped germanium, have been available with high performance in the 8 to 14 micron wavelength interval. These extrinsic photoconductors, however, require very low operating temperatures (below 30.degree. K). (Hg,Cd)Te intrinsic photodetectors having a spectral cutoff of 14 microns, on the other hand, are capable of high performance at 77.degree. K.
At the present time, most (Hg,Cd)Te is produced by bulk growth techniques such as the technique described by P. W. Kruse et al in U.S. Pat. No. 3,723,190. High quality (Hg,Cd)Te crystals are produced by this bulk growth technique.
Epitaxial growth techniques offer a number of potential advantages over bulk growth techniques. An epitaxial layer is a smooth continuous film grown on a substrate, such that the film crystal structure corresponds to and is determined by that of the substrate. The desired epitaxial layer is single crystal with uniform thickness and electrical property. The substrate has a different composition or electrical properties from that of the epitaxial layer.
A number of epitaxial growth techniques have been investigated in an attempt to grow (Hg,Cd)Te layers. Vapor phase epitaxial growth processes which have been studied are described in a number of patents including R. Ruehrwein (U.S. Pat. No. 3,496,024), G. Manley et al (U.S. Pat. No. 3,642,529), D. Carpenter et al (U.S. Pat. No. 3,619,283), R. Lee et al (U.S. Pat. No. 3,642,529), and R. Hager et al (U.S. Pat. No. 3,725,135).
Another epitaxial growth technique which has been investigated is liquid phase epitaxy ("LPE"). This technique is described in R. Maciolek et al (U.S. Pat. No. 3,902,924). Liquid phase epitaxial growth offers a number of advantages over both vapor phase epitaxial growth and bulk growth of (Hg,Cd)Te.
In a typical fabrication of mercury cadmium telluride epitaxial devices, individual devices are delineated by a process such as air abrasion or etching. Electrical contacts are then formed on the devices. The epitaxial layer and substrate are then bonded to a second substrate, which may be a "flat pack" which is subsequently attached to the cold finger of a Dewar or may be the Dewar itself.
(Hg,Cd)Te devices are typically used as photodetectors for infrared radiation. In order to obtain the desired sensitivity to infrared radiation, the (Hg,Cd)Te devices are operated at cryogenic temperatures in a Dewar. The operating temperatures are typically 77.degree. K or less and may, on occasion, be about 5.degree. K.
A problem has been encountered with the prior art epitaxial mercury cadmium telluride devices. The most common difficulty encountered in epitaxial (Hg,Cd)Te detector array development has been that the substrates sometimes cracked when cooled to cryogenic temperatures. At 77.degree. K cracking sometimes has occurred, and the problem has been even more pronounced for devices cooled to 5.degree. K.